Why does a pure solvent freeze faster than a solution?

Answer 1

Welcome to one of the most frustrating parts of physical chemistry. It's easy to visualize but difficult to explain. I'll try and make it simple.

Imagine all the molecules in a pure liquid moving around. When they solidify, they are able to form tentative "bonds" with other molecules around them. For this to start, they must form a critical mass. Naturally, with only the same type of molecules with which to interact, then it's not hard for nucleation (i.e., the making of mini-ice crystals) to occur.

The more nucleation, the faster the freezing. The nuclei eventually connect to make a bigger piece of ice (what you might have heard of as being referred to as a lattice).

SImilarly, the faster the rate - then the more nucleation is promoted, and the greater number of crystals of smaller size that will result.

Now, if you create a solution of which contains two or more substances, it becomes much more difficult for common molecules to find each other and soldify, forming ice. This causes the freezing temperature of the remaining solution to drop!

At temperatures well below the initial freezing point, some liquid water will remain because of the slowness of the interactions between common water molecules.

Also, unfrozen part of the solution will become more viscuous. The result is a decrease in the diffusion properties of the system and this will also slow down the crystallization.

The net consequence is that the solution will be much slower to freeze than the 'pure' water.

Answer 2

Because the solution has to be separated before it freezes, taking more time. It was worth a guess.

Answer 3

Because there are no other molecules in the way to slow down or prevent the phase transition.